In recent years, use of an LED in light sources, such as a general illumination light source and a light source of a backlight for a TV, is been rapidly promoted, as one of environmental countermeasures against global warming. Such light sources employ LED illumination modules configured to emit white light. How to generate white light by use of an LED can be exemplified by, for example, (i) a method in which white light is obtained by a combination of three primary colors of light, which are generated by respective three types of LEDs, namely, a red LED, a blue LED, and a green LED, and (ii) a method in which white light is generated by mixing blue light and yellow light by employing a blue LED as a light source with a yellow fluorescent material. In a field of an LED illumination module, such developments are being made as a development of a high-luminance module and a development of a technique for realizing a light-adjustment function by use of a uniqueness of an LED that a conventional light source does not have.
For example, Patent Literature 1 discloses a technique for generating white light with a light rendering property. According to Patent Literature 1, a composition containing a green fluorescent material and a red fluorescent material, both of which can be excited by a single shared blue LED element, is provided so as to cover the blue LED element, and the blue LED is caused to emit blue light. In the arrangement, (i) the blue light is partially wavelength-converted into green light by the green fluorescent material, and (ii) the blue light is partially wavelength-converted into red light by the red fluorescent material. The green light, the red light, and the blue light are mixed with each other in accordance with additive color mixture, thereby generating white light with a light rendering property.
Further, Patent Literature 2 discloses a technique for generating white light having a desired color tone. According to Patent Literature 2, (i) three types of LED elements, namely, a red LED element which emits red light, a green LED element which emits green light, and a blue LED element which emits blue light, are sealed together in a resin film, and (ii) the three types of LED elements are simultaneously turned on in the resin film while respective amounts of the red light, the green light, and the blue light are adjusted independently, so that they are mixed with each other in accordance with additive color mixture. Thus, white light having a desired color tone is generated.
However, in such a technique as disclosed in Patent Literature 1 that employs different types of fluorescent materials, reabsorption of light occurs between the different fluorescent materials. This makes it difficult to optimize a white light spectrum. Further, in such a technique as disclosed in Patent Literature 2 that causes three types of LED elements to be simultaneously turned on, even if it seems, to the eyes, that white light is obtained, an illuminated object cannot be observed naturally in some cases. This is because, in terms of a spectrum, the white light has a wavelength band having no radiation energy. In view of this, it is difficult to achieve a high color rendering property by the techniques disclosed in Patent Literature 1 and 2.
Meanwhile, Patent Literature 3 discloses a technique for improving the color rendering property by additionally mixing yellow light with red light, green light, and blue light, for example. In a light emitting device disclosed in Patent Literature 3, (i) a red LED element, a green LED element, a blue LED element, and a yellow LED element are provided on a single substrate, and collectively sealed with a translucent resin, and (ii) each of the LED elements is independently driven. This arrangement makes it possible to generate white light that can be adjusted to a given color, while maintaining the color rendering property and color reproducibility.
In the technique disclosed in Patent Literature 3, the color rendering property is improved as compared with the techniques disclosed in Patent Literatures 1 and 2. However, in the technique disclosed in Patent Literature 3, the color rendering property is still low because the blue light, emitted from the blue LED element, is reabsorbed by the red LED element and the green LED element.
On the other hand, in such a method in which white light is generated by mixing blue light and yellow light by use of a blue LED as a light source of a yellow fluorescent material, pseudo white light that is wholly yellowish is generated due to insufficiency of a red-light emission component. For this reason, it is impossible to achieve a high color rendering property by use of the method. However, several methods have been developed to compensate such a red-light emission component that tends to be insufficient.
For example, Patent Literature 4 discloses such a technique that (i) a plurality of white LED devices each of which emits pseudo white light and a plurality of red LED devices each of which emits red light are arranged on a single substrate, (ii) a lens serving as color mixing means is provided above these LED devices, and (iii) the plurality of white LED devices and the plurality of red LED devices are driven independently so as to adjust a tone of white light emitted via the lens.